Piezoelectric polymers are utilised in particular to produce electromechanical transducers, such as transducers transmitting and receiving sonic and ultrasonic radiations.
In many applications, these macromolecular materials having a polar orientation are advantageously substituted for piezoelectric ceramics since they have the advantage of a lower Young's modulus, a lesser volumetric mass and a characteristic impedance closer to that of aqueous and biological environments. The piezoelectric coefficients are distinctly lower than those of the known piezoelectric ceramic materials, but overall performance factors of interest are reached by application of an energetic electric polarisation combined, if appropriate, with an initial mechanical treatment. An important feature of the application of piezoelectric polymers is their ease of application if there is a need for transducer elements of large area. It should equally be pointed out that the forming to shape of such elements is easy, but the most current thicknesses produced remain between a few microns and one to two millimeters.
The electric polarising field to be produced within a polymer so as to cause a conversion within the same, giving rise to the piezoelectric, electro-optical or pyro-electric properties sought, is between 5.times.10.sup.7 and 1.5.times.10.sup.8 volts per meter.
A high tension source is thus commonly required to polarise the polymer, and if it is intended to polarise a sample of a thickness of the centimeter in a single operation, the voltage which has to be applied is of the order of a million volts, which is prohibitive. The trend of the applications of piezoelectric polymers towards massive structures in the form of plates or blocks makes it possible to have available a greater number of parameters for appropriate location of a range of operating frequencies or for increasing the sensitivity of a pickup device for vibrational radiation. Moreover, as soon as the optimum dimensioning of a transducer necessitates choosing a thickness of the order of a millimeter, a production problem intervenes for which the solution cannot consist in simply stacking films of lesser thickness. As a matter of fact, a volume of solid material produced by stacking sheets is not mechanically equivalent to the same volume produced in one piece. It is well known that a stack of thin laminae does not withstand bending under load as would a massive plate having the thickness of the stack. Similarly, it is equally known that within a stack, each sheet may be considered as a source radiating through the adjacent sheets and that any impedance breakdown between sheets may cause undesirable reflections and losses by dissipation of power.
In order to eliminate the disadvantages listed in the foregoing, the invention proposes that a production technique be adopted which has been utilised to produce plastic film capacitors in the "slab" or "flattened coil" configurations.